High five for playing Warband :D
Before continuing playing on your save, I would highly recommend you to download the Diplomacy mod. It adds a lot more features, well, diplomacy-wise and can be quite useful once you start your own kingdom.
As for your questions:
- I'm not sure how much right of rule you need but I do know that you need a lot in order to not be hated by the other factions. One easy way to gain right of rule is sending your companions on missions. Once you get enough, just attack a faction and take one of their castles/towns and you'll set up your faction. That's all I know about creating your own kingdom... never really explored it that much ^_^;
- Your army seems good enough, a little mixture of both. I usually role out for Vaegirs archers, Nords Infantry and Swadian knights as they are the best of the best troops in vanilla Warband and don't bother with the rest. Though Heavy cav is pretty broken in vanilla Warband...
- In general, Swadians have the best heavy cav, Vaegirs have the best archers, and Nords have best infantry. The Sarranids troops are more balanced than the other factions. Rhodoks are alright. They are much better at defending castles than on the field (unless it's against cav). Khergit's is meh for me. They have horse archers and are all mounted but once their horse is taken down, their dead meat. In addition, they are terrible in sieges.
- For sieges, I usually have my archers stay on the ground while both infantry and cavalry charge up the ladder/siege tower. I usually stay with the archers and snipe enemy archers with my bow. Once I take the ledge, I would be my archers up and have them continue to support my infantry and cavalry. On what troops to bring to sieges, I highly recommend taking either Rhodok Sergent or Nord Huskarls as your main infantry unit while you have a bunch of archers (preferably Vaegirs archers) backing them up. Seiges aren't all that hard in vanilla warband... *shrugs*
LOL, as usual, the Swadians are getting butt kicked hard by Rhodoks.